This invention relates to differential amplifier output leading circuits, and more particularly to a differential amplifier output leading circuit which is employed in the IF (intermediate frequency) amplifier stage of a tuner.
The IF amplifier of an FM tuner is, in general, designed so that differential amplifiers having a current limiter characteristic are cascade-connected in several stages, and the differential outputs of the differential amplifier of the last stage are applied to an FM detector circuit in the following stage. The outputs of these differential amplifiers are subjected to AM detection, to obtain an IF level signal meter driving signal. The signals for AM detection are provided by leading one of the differential outputs of each differential amplifier, and accordingly the loads of the pair of differential outputs of each differential amplifier are unbalanced. On the other hand, the FM detector circuit is designed so that it receives the balanced outputs of the IF amplifier. However, the balanced output is substantially unbalanced by the unbalanced loads of the differential amplifier described above. As a result, the AMR (amplitude variation elimination ratio) becomes considerably worse.
The differential outputs of each differential amplifier are applied through buffer circuits, or emitter follower circuits, to another differential amplifier in the next stage or to the FM detector circuit, and the load of each emitter follower circuit is a resistance load. Accordingly, the signal current flows through the resistance load to the reference voltage line, thus adversely affecting other circuits. In the case where the IF amplifier, the FM detector circuit, and other relevant circuits are integrated on a single semiconductor chip, this bad influence cannot be neglected; that is, the circuit becomes unstable, the AMR becomes worse, and the signal is distorted. FIG. 1 shows the signal characeristics of a multistage type IF amplifier and an FM detector circuit. In FIG. 1, the curves 11, 12, 13 and 14 indicate the variations of the FM detection level, AMR, the signal level and the signal distortion with respect to input levels, respectively. As is apparent from FIG. 1, the conventional circuit is disadvantageous in that, for the aforementioned reasons, the characteristics greatly vary with respect to the input signal levels.